Many high speed shrink wrapping machines are designed to form an overlapping seam at the bottom of the package which is being wrapped. The seam may be made by wrapping the film about the package in tubular form so that the opposite longitudinal edges overlap. In many cases, the overlapping portions need only be retained in overlapping relation long enough for the package to pass through a shrink tunnel; hence, a permanent seal is not required.
One way of making such an overlapping seam is to employ static seam-forming apparatus wherein the seam is made by placing the overlapping portions of the web or film in contact with each other between a static charging head and a grounded backing plate and generating a high voltage DC static charge between the head and the plate so that the overlapping portions of the film are oppositely charged and drawn together. A common problem encountered in the use of such apparatus is that the staticized film tends to stick to the backing plate, causing the film to mistrack or lose track due to the resistance of movement of the film over the backing plate. Another problem is the entrapment of bubbles of air between the overlapping portions of the film which weakens the seam and is unattractive in that it reduces the transparency of the seam. The foregoing problems derive particularly from the use of a multi-electrode static head.
It is the purpose of this invention to provide an apparatus and method for reducing and substantially eliminating the resistance to movement of the overlapping portions of the film as the seam is moved through the static sealer and to substantially eliminate the entrapment of air bubbles between the overlapping portions of the film.